Networks typically employ one or more servers that store files and other information accessible by several work stations that are part of the network through a switch. When the server sends a particular file to a station, the TCP separates the file into data packets and transmits the data packets in groups referred to as TCP windows. The size of the TCP window determines how much data can be in flight between the server and a station at any given time. By defining TCP windows to include a plurality of data packets, the throughput can be increased because the station does not transmit an acknowledgment back to the server that it received the data packets until it has received all of the data packets in the TCP window, as opposed to sending an acknowledgment back each time it receives a data packet.
The size of the data packets will typically be about 1500 bytes. Certain known systems employ a 64 k byte TCP window size. Therefore, known TCP windows typically include about 42 data packets. Each data packet in a TCP window has a header that may include a 16-bit field that identifies how big the TCP window can be.
Typically, the server will be operating at a much faster speed than the stations. For example, the server may operate at 1 giga-bit/per second and the stations may operate at 100 mega-bits/per second. The server will send each TCP window at its faster speed that is received by the switch. The switch will then send the TCP windows to the appropriate station at the slower station speed. Because the switch is receiving the data packets from the server much faster than it is transmitting the data packets to the work station, the data packets are stored in a buffer in the switch until they can be transmitted to the station.
Because of the speed mismatch between the server and the stations, it is possible that data packets will be dropped as a result of the buffer being too small. Particularly, if the buffer is smaller than the TCP window size, a dropped packet may occur. The speed difference between the server and the stations and the interaction between the TCP and the SMB protocols combined with the size of the buffer influences the number of dropped packets that may affect system performance. If a dropped packet happens to be at the end of an upper layer protocol boundary, significant upper level protocol dependent time-out delays can occur resulting in poor throughput performance.
If a data packet does happen to be dropped, the TCP recognizes this because the station will not send an acknowledgment message back to the server acknowledging that it received all of the packets in the TCP window that were transmitted. When the server identifies a dropped packet, it will then resend the file starting with the dropped data packet.
One possible solution to the dropped packet problem referred to above is to provide more memory in the buffer, so that all of the data packets received by the switch from the server have a place to be stored before they are transmitted to the station. However, this solution has significant drawbacks because of the cost of providing so much memory to eliminate the problem. For example, it is desirable that the buffer memory be provided on the same application specific integrated circuit chip (ASIC) within the switch that includes the processor. If more memory is required beyond what can fit on the ASIC, and the ASIC has to connect to other chips having the additional memory, then the cost of the switch goes up considerably.
Another solution to the problem referred to above is to reconfigure the operating system, such as Windows™, to reduce the size of the TCP windows that it receives. This solution requires significant software knowledge, and is not trivial to perform, and therefore, it is generally not a viable solution.